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HL
1
31 tháng 1
\(A=1\cdot2+2\cdot3+...+151\cdot152\)
\(=1\left(1+1\right)+2\left(1+2\right)+...+151\left(1+151\right)\)
\(=\left(1+2+3+...+151\right)+\left(1^2+2^2+...+151^2\right)\)
\(=\dfrac{151\left(151+1\right)}{2}+\dfrac{151\left(151+1\right)\left(2\cdot151+1\right)}{6}\)
\(=151\cdot76+\dfrac{151\cdot152\cdot303}{6}\)
\(=151\cdot76+151\cdot7676=1170552\)
\(C=2\cdot4+4\cdot6+...+2024\cdot2026\)
\(=2\cdot2\left(1\cdot2+2\cdot3+...+1012\cdot1013\right)\)
\(=4\left[1\left(1+1\right)+2\left(1+2\right)+...+1012\left(1+1012\right)\right]\)
\(=4\left[\left(1+2+...+1012\right)+\left(1^2+2^2+...+1012^2\right)\right]\)
\(=4\left[1012\cdot\dfrac{1013}{2}+\dfrac{1012\left(1012+1\right)\left(2\cdot1012+1\right)}{6}\right]\)
\(=4\left[506\cdot1013+345990150\right]\)
\(=1386010912\)
\(M=1^2+2^2+...+2024^2\)
\(=\dfrac{2024\left(2024+1\right)\cdot\left(2\cdot2024+1\right)}{6}\)
\(=2024\cdot2025\cdot\dfrac{4049}{6}\)
=2765871900
\(N=1^3+2^3+...+100^3\)
\(=\left(1+2+3+...+100\right)^2\)
\(=\left[\dfrac{100\left(100+1\right)}{2}\right]^2\)
\(=\left[50\cdot101\right]^2=5050^2\)
\(Q=1^3+2^3+...+2024^3\)
\(=\left(1+2+3+...+2024\right)^2\)
\(=\left[\dfrac{2024\left(2024+1\right)}{2}\right]^2\)
\(=\left[1012\left(2024+1\right)\right]^2\)
\(=2049300^2\)
TT
1
12 tháng 3 2018
BÀI 1:
\(S=1+\frac{1}{2}+\frac{1}{4}+\frac{1}{8}+\frac{1}{16}+\frac{1}{32}\)
\(S=1+\frac{1}{1.2}+\frac{1}{2.2}+\frac{1}{2.4}+\frac{1}{4.4}+\frac{1}{4.8}\)
\(S=1+1-\frac{1}{2}+\frac{1}{2}-\frac{1}{2}+\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{4}+\frac{1}{4}-\frac{1}{8}\)
\(S=1+1-\frac{1}{8}\)
\(S=\frac{15}{8}\)
BÀI 2:
\(A=1.2+2.3+3.4+...+98.99\)
\(\Rightarrow3A=1.2.3+2.3.3+3.4.3+...+98.99.3\)
\(3A=1.2.\left(3-0\right)+2.3.\left(4-1\right)+3.4.\left(5-2\right)+...+98.99.\left(100-97\right)\)
\(3A=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+98.99.100-97.98.99\)
\(3A=\left(1.2.3+2.3.4+3.4.5+98.99.100\right)-\left(1.2.3+2.3.4+...+97.98.99\right)\)
\(3A=98.99.100\)
\(3A=970200\)
\(\Rightarrow A=970200:3\)
\(A=323400\)
CHÚC BN HỌC TỐT!!!
PN
1
27 tháng 4 2018
2 ) 1.2 + 2.3 + 3.4 + ... + x.( x + 1 ) = 5850
Đặt A = 1.2 + 2.3 + 3.4 + ... + x.( x + 1 )
=> 3A = 1.2.3 + 2.3.( 4 - 1 ) + 3.4.(5 - 2 ) + ... + x.( x + 1 ). [ ( x + 2 ) - ( x - 1 ) ]
= 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ... + x . ( x + 1 ) . ( x + 2 ) - ( x - 1 ). x . ( x + 1 )
= x . ( x + 1 ) . ( x + 2 )
=> A = [ x . ( x + 1 ) . ( x + 2 ) ] : 3
Mà tổng A = 5850
=> [ x . ( x + 1 ) . ( x + 2 ) ] : 3 = 5850
=> x . ( x + 1 ) . ( x + 2 ) = 17550
x . ( x + 1 ) . ( x + 2 ) = 25 . 26 . 27
=> x = 25
Vậy x = 25
PM
2
VD
23 tháng 1 2022
3C=1.2.3+2.3.(4-1)+3.4.(5-2)+...+2014.2015.(2016-2013)
3C=2014.2015.2016
C=2014.2015.2016:3
TT
4
9 tháng 6 2016
Khó quá mk chịu thôi bạn giải giùm mk nha
k mình đi
kb luôn nhé
TT
1
25 tháng 8 2017
Ta có : S = 1.2 + 2.3 + 3.4 + ..... + 32.33
=> 3S = 1.2.3 - 1.2.3 + 2.3.4 - 2.3.4 + ...... + 32.33.34
=> 3S = 32.33.34
=> S = \(\frac{32.33.34}{3}=11968\)
6 tháng 7 2019
a) Đặt A = 1/2 + 1/4 + 1/8 + 1/16 + 1/32
A = 1/2 + 1/22 + 1/23 + 1/24 + 1/25
2A = 2(1/2 + 22 + 1/23 + 1/24 + 1/25)
2A = 1 + 1/2 + 1/22 + 1/23 + 1/24
2A - A = (1 + 1/2 + 1/22 + 1/23 + 1/24) - (1/2 + 1/22 + 1/23 + 1/24 + 1/25)
A = 1 - 1/25
A = 31/32
b) 2/1.2 + 2/2.3 + 2/3.4 + ... + 2/18 . 19 + 2/19.20
= 2(1/1.2 + 1/2.3 + 1/3.4 + ... + 1/18.19 + 1/19.20)
= 2.(1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/18 - 1/19 + 1/19 - 1/20)
= 2. (1 - 1/20)
= 2.19/20
= 19/10
CQ
8
LT
15 tháng 9 2016
A = 1.2 + 2.3 + 3.4 + ...+ 59.60
3A = 1.2.3 + 2.3.3 + 3.4.3 + ...+ 59.60.3
3A = 1.2.(3-0) + 2.3.(4-1) + 3.4.(5-2) +...+ 59.60.(61-58)
3A = 1.2.3 - 0 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 +...+ 59.60.61 - 58.59.60
3A = 58.59.60 => A = 58.59.60 : 3 = 68 440
B = 12 + 22 + 32 + 592
B = 1.2 + 2.2 + 3.3 + 59.59
B = 2 + 4 + 9 + 3481
B = 3496
vậy A - B = 68 440 - 3 496 = 64 944
( bấm nhé )
\(x=1.2+2.3+3.4+...+99.100\)
\(x=1.\left(1+1\right)+2.\left(2+1\right)+3.\left(3+1\right)+...+99.\left(99+1\right)\)
\(x=1^2+1.1+2^2+2.1+3^2+3.1+...+99^2+99.1\)
\(x=y+\left(1+2+3+...+99\right)\)
\(x=y+\frac{99.\left(99+1\right)}{2}=y+4950\)
\(x-y=4950\)